For a long while, I’ve wanted a low voltage AC power source for my shop bench. I have a variac, but it’s too easy for someone to go overvoltage with it. Its output is not isolated from mains, and it has no voltmeter or ammeter. So, we haul out a transformer with the right output voltage, wire a mains cord and plug to the input, use alligator leads on the output and go. Not convenient or particularly safe. I came up with a design with several fixed voltages selected with a rotary switch, but a completely variable voltage is more flexible. I thought about getting another variac and mechanically limiting how far the knob could be turned. But discarded this idea because I acquired a new Powerstat variable transformer with 40 VAC input and 0-40 VAC output. Using a 120 VAC to 40 VAC input transformer for it will give AC isolation as well. The preliminary design, which is simple, is in the pic below.
In addition to the power switch, there is an output switch. I think it’s a good idea for bench PSUs to have an output switch. It can avoid frying something inadvertently when the PSU is switched on. I used a 3-pole output switch with one pole used to control power to the ammeter. The ammeter needs 9 VDC to operate, including the light for its display. This will provide an obvious visual indication of the switch position.
Comments welcomed.
Mike
Added note: Displayed voltage would be a little more accurate if the voltmeter was connected to the “output” end of the shunt resistor.
Edit: Replaced original layout because the transformer was shown wired with a short!